PRODUCTION OF A MATERIAL COMPRISING A MIXTURE OF NOBLE METAL NANOPARTICLES AND RARE-EARTH OXIDE NANOPARTICLES
The invention relates to the production of a material comprising a mixture of noble metal nanoparticles and rare-earth oxide nanoparticles. The process comprises the following successive steps: a) production of a metal alloy comprising at least one noble metal chosen from the group comprising the elements Ru, Rh, Ir, Ag, Au, Pd, Pt, Ni and Cu and at least one rare earth chosen from the group comprising the elements La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y and Sc, said alloy containing a crystalline phase the rare earth content of which is greater than 10 at % and the noble metal content of which is between 25 and 75 at %; and b) oxidation, in an oxidizing atmosphere, of the metal alloy obtained during step a). The subject of the invention is also a composite comprising a mixture of noble metal nanoparticles and rare-earth oxide nanoparticles and to the use of such a composite, in particular for catalysis.
The present invention relates to a process for producing a material comprising a mixture of noble metal nanoparticles and rare-earth oxide nanoparticles, to a composite comprising such a mixture and to various uses of this composite.
Numerous fields require the use of materials comprising nanoscale noble metal particles and oxide particles. Such fields include catalysis, optics, magnetism and powder metallurgy.
In particular, noble metals such as gold or palladium are known for their advantageous properties as catalysts. Their catalytic activity is particularly exacerbated when they are in the form of nanoparticles supported on an oxide.
FR 2 779 666 teaches a process for producing materials comprising noble metal nanoparticles and nanoparticles of oxides of a reducing metal, the reducing metal being chosen from column IVB of the Periodic Table of the Elements, namely from titanium, zirconium and hafnium.
However, trials carried out by the inventors have shown that among the numerous noble metal/reducing metal pairs described in that document, only the pair Au/Zr does actually allow a material comprising noble metal nanoparticles and reducing-metal oxide nanoparticles to be obtained. Thus, it seems that the above document cannot reasonably be seen as providing relevant teaching for forming a material comprising noble metal nanoparticles and oxide nanoparticles.
The inventors have now discovered that, by replacing all or part of a non-noble metal (or reducing metal) as indicated in FR 2 779 666 with a rare earth, and by forming a noble metal/rare earth metal alloy having a crystalline phase the composition of which satisfies specified criteria, it is possible to obtain the expected result in terms of structure of material, irrespective of the alloy used.
Thus, according to a first aspect, one subject of the invention is a process for producing a material comprising a mixture of noble metal nanoparticles and rare-earth oxide nanoparticles, comprising the following successive steps:
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- a) production of a metal alloy comprising at least one noble metal chosen from the group comprising the elements Ru, Rh, Ir, Ag, Au, Pd, Pt, Ni and Cu and at least one rare earth chosen from the group comprising the elements La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y and Sc, said alloy containing a crystalline phase the rare earth content of which is greater than 10 at % and the noble metal content of which is between 25 and 75 at %; and
- b) oxidation, in an oxidizing atmosphere, of the metal alloy obtained during step a).
Such a process makes it possible to obtain, in a well-controlled manner, a material comprising nanoparticles of different types, namely noble metal nanoparticles and nanoparticles of a rare-earth oxide.
According to a first embodiment, the material obtained is a binary material in which all the noble metal nanoparticles consist of the same noble metal or of an alloy of noble metals, and all the rare-earth oxide nanoparticles contain the same rare earth.
According to a second embodiment, the process according to the invention enables a ternary, quaternary or higher-order material to be obtained. In such a material, the noble metal nanoparticles are formed by a mixture of nanoparticles of different noble metals (for example a mixture of nanoparticles of a noble metal NM1 and nanoparticles of a noble metal NM2) and/or by nanoparticles of an alloy of noble metals (NM1/NM2 nanoparticles), and the rare-earth oxide nanoparticles are formed by a mixture of nanoparticles of oxides of different rare earths (for example a mixture of nanoparticles of the oxide of a rare earth RE1 and nanoparticles of the oxide of a rare earth RE2) and/or by nanoparticles of an alloy of oxides depending on the addition elements comprising at least one rare-earth oxide.
According to one embodiment, the alloy produced during step a) may further include at least one transition metal chosen from the group comprising the elements of column IVB of the Periodic Table of the Elements: Ti, Zr, Hf, from column VB; V, Nb and Ta from column VIB; Cr, Mo and W from column VIIB; Mn, Tc and Re from column IIB; and Zn, Cd and Hg, and the elements Fe, Co and Os.
It is thus possible to obtain a ternary material in which all the noble metal nanoparticles consist of the same noble metal, all the rare-earth oxide nanoparticles contain the same rare earth and all the transition metal oxide nanoparticles contain the same transition metal.
It is also possible to obtain a quaternary or higher-order material. In such a material, the noble metal nanoparticles are formed by a mixture of nanoparticles of different noble metals (for example a mixture of nanoparticles of a noble metal NM1 and nanoparticles of a noble metal NM2), the rare-earth oxide nanoparticles are formed by a mixture of nanoparticles of the oxides of different rare earths (for example a mixture of nanoparticles of the oxide of a rare earth RE1 and nanoparticles of oxides of a rare earth RE2) or by nanoparticles containing an alloy of several rare earths, and the transition metal oxide nanoparticles contain a mixture of oxides of different transition metals (for example a mixture of nanoparticles of the oxide of a transition metal TM1 and nanoparticles of the oxide of a transition metal TM2) or an alloy of several transition metals.
When such hybrid composites are used in catalysis, their catalytic activity may prove to be superior to that of a simple composite containing only a single type of oxide forming a support for the noble metal.
Provision may also be made, in the alloy produced during step a) for the rare earth to be partially replaced with an element of the actinide family chosen from Ac, Th and Pa.
Step a) of the process according to the invention, which consists in producing the metal alloy, may be carried out by various methods known to those skilled in the art. For example, this step may be carried out by melting the pure elements, for example in an arc furnace, or by powder metallurgy or thin films heated to a temperature of 200° C. or higher, or else by mechanical synthesis from the pure elements or alloys, carried out at low temperature and preferably at ambient temperature.
When step a) is carried out at a temperature above 50° C., it is preferably performed in an inert or reducing atmosphere so as to prevent the alloy from oxidizing.
Step b) of the process according to the invention, which consists in oxidizing the alloy produced during step a), is preferably carried out at a temperature below 800° C.
According to one embodiment, step b) is carried out at ambient temperature and may also be carried out in air.
Moreover, between steps a) and b), a heat treatment step may be provided for heating the metal alloy to a temperature between 200° C. and 1000° C. in an inert or reducing atmosphere. Such a step makes it possible to obtain various microstructures of the metal alloy formed during step a).
It is also possible to provide, between steps a) and b), and optionally as a complement to the abovementioned heat treatment step, a step of grinding the metal alloy, intended to increase the rate of oxidation during step b).
Furthermore, the process according to the invention may include, after step b) a step of mechanically grinding or ultrasonically treating the powder obtained, this step being intended to modify (where appropriate, to decrease) the size of the particles obtained.
It is also possible to provide, after step b), a coalescence heat treatment step intended to adjust the size of the particles obtained. This step may also be advantageously combined with step b) of oxidizing the metal alloy, by choosing the oxidation temperature appropriately. The temperature used during this coalescence heat treatment step depends on the constituent elements of the alloy and must be chosen in particular so as not to exceed the melting point of each element.
According to a second aspect, the invention relates to a composite comprising a mixture comprising, on the one hand, nanoparticles of at least one noble metal chosen from the elements Ru, Rh or Ir, Ag, Au, Pd, Pt, Ni and Cu, it being understood that the noble metal nanoparticles may all consist of the same noble metal or else consist of a mixture of nanoparticles of different noble metals (for example a mixture of nanoparticles of a noble metal NM1 and nanoparticles of a noble metal NM2) and/or by nanoparticles of an alloy of noble metals (NM1/NM2 nanoparticles), and, on the other hand, nanoparticles of at least one oxide of a rare earth, said rare earth being chosen from the elements La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y and Sc, it being understood that the nanoparticles of the oxide of a rare earth may all contain the same rare earth, or else they may be formed by a mixture of nanoparticles containing different rare earths (for example a mixture of nanoparticles of the oxide of a rare earth RE1 and nanoparticles of the oxide of a rare earth RE2) and/or by nanoparticles of an alloy of oxides of several rare earths and/or an alloy of transition metals depending on the addition elements, said nanoparticles having a particle size of less than 20 nm. This composite has the particular feature of having a high percentage content of noble metal by weight, equal to or greater than 20%.
The composite according to the invention takes the form of a powder containing porous agglomerates, the size of which varies from one micron to a few hundred microns, the agglomerates themselves consisting of an intimate mixture of particles and optionally of noble metal and rare-earth oxide wires, with a size of less than 20 nm.
Such composites have a high specific surface area, of around 60 m2/g, and a high concentration of noble metal, making them particularly advantageous in particular for use in the field of catalysis.
The composite of the invention may further include nanoparticles of at least one oxide of a transition metal, said transition metal being chosen from the group comprising the elements of column IVB of the Periodic Table of the Elements: Ti, Zr, Hf, from column VB; V, Nb and Ta from column VIB; Cr, Mo and W from column VIIB; Mn, Tc and Re from column IIB; and Zn, Cd and Hg, and the elements Fe, Co and Os, it being understood that the transition metal oxide nanoparticles may all contain the same transition metal, or else they may consist of a mixture of nanoparticles of the oxides of different transition metals (for example a mixture of nanoparticles of the oxide of a transition metal TM1 and nanoparticles of the oxide of a transition metal TM2) or by nanoparticles containing an alloy of several transition metals and/or rare earths.
The composite according to the invention may also include nanoparticles of at least one oxide of an element from the actinide family, said element being chosen from Ac, Th and Pa.
As indicated above the composites according to the invention may advantageously be used in the field of catalysis, as some of the following examples will show. They may also be used in other fields, such as the manufacture of nonlinear optical instruments or the production of nanoscale oxide powders, for example for the manufacture of sintered ceramics.
The present invention is illustrated below by specific examples of its implementation, to which however it is not limited.
All the metal alloys presented in these examples were synthesized by melting them in an arc furnace in an argon atmosphere.
The materials were characterized in particular by X-ray diffraction (Co—Kα and Cu—Kα), the diffraction spectra showing intensity I, in arbitrary units, as a function of the diffraction angle 2θ. The particle size was estimated using the Scherrer equation.
Examples 1 to 13 and 21 to 25 describe the production and characterization of materials obtained according to the process of the invention and the catalytic activity of some of them. The results obtained in simple oxidation of CO are shown in
Examples 14 to 20 describe three series of comparative experiments, each series having an example of a binary alloy corresponding to the definition given in document FR 2 779 666 and one or two examples of a ternary alloy obtained by the addition of cerium to the binary alloy, according to the process of the invention.
For these examples, the oxidation was carried out during the thermogravimetric analysis of the alloy carried out in air from 25° C. to 800° C. at a heating rate of 10° C./min.
The figures that show the results of the thermogravimetric analysis are each expressed as three ordinates: the thermal flux F in μV.S/mg; the temperature T in degrees Celsius; and the mass M in mg as a function of the time t in seconds. In each of these figures, the curves denoted by (a), (b) and (c) correspond to the change in thermal flux F, to the change in temperature T and to the change in mass M respectively.
EXAMPLE 1 Au/CeO2 CompositeProduction
The equiatomic Ce/Au alloy was synthesized from the elements Au and Ce of purity close to 99%. The synthesis was carried out in a water-cooled crucible so as to prevent contamination at the alloy. The alloy was then coarsely ground, so as to obtain a particle size varying from a few tens of microns to a few hundred microns, and then oxidized by exposure to air with a relative humidity of 60% and at a temperature of 60° C.
Characterization
The powder obtained was characterized by X-ray diffraction (
The micrograph of
The volume expansion that occurs during the alloy oxidation step results in a division of the agglomerates, thereby explaining the large size distribution, and at the same time leads to the formation of nanoparticles visible by transmission electron micrography.
The morphology of the powder obtained was confirmed by the high value of the specific surface area. From the curve shown in
The percentage content of gold by weight in the material was 53.4%.
Catalytic Activity
The catalytic activity of the powder obtained was examined by measuring the degree of conversion of CO as a function of temperature.
Production
The ternary alloy Zr0.5Ce0.5Au was synthesized from the elements Au, Zr and Ce of purity close to 99%. The metal alloy obtained was multiphased and composed predominantly of a ZrAu phase and a CeAu phase. The metal alloy, after coarse grinding, was oxidized in air at ambient temperature.
An additional treatment step was carried out on the powder by ultrasound (20 kHz for 10 minutes).
This additional treatment can be used to obtain one particular form of the composite, required for the envisaged application. It may also advantageously be used to control the size of the nanoparticles obtained, in particular for use in liquid-phase catalysis for which the recommended size may be greater than 20 nm (fine chemistry).
Characterization
The powder obtained was characterized by transmission electron micrography (
The analysis by transmission electron micrography carried out before the ultrasonic treatment step shows that the metal phase (Au) is very widely dispersed (
The additional ultrasonic treatment step carried out on the powder allowed the agglomerates consisting of gold nanoparticles and oxide nanoparticles to be fragmented and to be given a uniform size of around 20 μm (
The morphology of the powder obtained was confirmed by the high value of the specific surface area (63.8 m2/g) measured by the BET method.
The percentage content of gold by weight in the composite was 57.2%.
Catalytic Activity
The catalytic activity was measured for a reaction mixture consisting of 1.72% CO and 3.7% O2 in nitrogen with a flow rate of 26 ml/min for the simple oxidation and 1.56% CO, 3.3% O2 and 10% H2 in nitrogen with a flow rate at 29 ml/min for the selective oxidation of CO in the presence of hydrogen. The tests were carried out with 8 mg of powder mixed with about 800 mg of alumina (Al2O3). The alumina was used here as diluent as it does not have a catalytic activity.
It is apparent that the catalytic activity for CO oxidation (
Production
The ternary alloy of Zr0.75Ce0.25Au composition was synthesized from the elements Au, Zr and Ce of purity close to 99%. The metal alloy, after coarse grinding, was oxidized in air at a temperature of 80° C.
Characterization
The characterization of the metal precursor and then of the powder obtained after oxidation was carried out by X-ray diffraction.
The metal alloy was composed predominantly of ZrAu and (Ce,Zr)9Au11 phases. In the latter phase, Zr partially substitutes for Ce in the Ce9Au11 phase.
The specific surface area measured by the BET method was 58.2 m2/g.
The percentage content of gold by weight in the composite was 59.3%.
Catalytic Activity
The catalytic tests were carried out under the same conditions as in Example 2. It is apparent that the catalytic activity of the material obtained during oxidation of CO (
Production
The ternary alloy of Zr0.25Ce0.75Au composition was synthesized from the elements Au, Zr and Ce with a purity close to 99%. The metal alloy, after coarse grinding, was oxidized in air at ambient temperature.
Characterization
The characterization of the metal precursor and then of the powder obtained after oxidation was carried out by X-ray diffraction.
The metal alloy is predominantly composed of the CeAu phase (about 80%) and the ZrAu phase.
The powder obtained after oxidation is composed of gold particles with a size close to 7 nm and ceria CeO2 and zirconia (ZrO2) particles, again of nanoscale size.
The percentage content of gold by weight in the composite was 55.2%.
Catalytic Activity
The catalytic tests were carried out under the same conditions as in Example 2.
In the case of selective CO oxidation in the presence of hydrogen, the maximum conversion is obtained at close to 60° C. (
Production
The binary alloy of Ce0.5Pd0.5 composition was synthesized from the elements Pd and Ce with a purity of close to 99%. The metal alloy, after coarse grinding, was oxidized in air at a temperature of 80° C. and with a relative humidity of 100%.
Characterization
The characterization of the metal precursor and of the powder obtained after oxidation was carried out by X-ray diffraction.
The metal alloy was a single-phase alloy consisting of the CePd phase.
The percentage content of palladium by weight in the composite was 38.2%.
Catalytic Activity
The catalytic tests were carried out under the same conditions as in Example 2. Although the size of the palladium particles was less than 10 nm, the Pd/CeO2 composite was active only at high temperature (220° C.) in CO conversion (
Production
The binary alloy of Ce0.5Pt0.5 composition was synthesized from the elements Pt and Ce of purity close to 99%. The metal alloy, after coarse grinding, was oxidized in air at a temperature of 80° C. and with a relative humidity of 100%.
Characterization
The characterization of the metal precursor and then of the powder obtained after oxidation was carried out by X-ray diffraction.
The metal alloy was a single-phase alloy (CePt phase).
The powder obtained after oxidation was made up of platinum particles with a size of less than 10 nm and ceria (CeO2) particles, again of nanoscale size.
The percentage content of platinum by weight in the composite was 53.1%.
Catalytic Activity
The catalytic tests were carried out under the same conditions as in Example 2. The complete conversion temperature was 170° C. (
Production
The ternary alloy of Ce0.5Pt0.1Au0.4 composition was synthesized from the pure elements Au, Pt and Ce with a purity close to 99%. The metal alloy, after coarse grinding, was oxidized in air at a temperature of 100° C. with a relative humidity of 100%.
Characterization
The characterization of the metal precursor and then of the powder obtained after oxidation was carried out by X-ray diffraction (Co—Kα). The metal alloy was predominantly composed of the CeAu phase, the only one detected by X-ray diffraction. Given the high solubility of platinum in gold, it is consistent to find gold substituted with platinum in the CeAu phase, giving rise to the ternary compound Ce(AuPt).
The percentage content of noble metal by weight in the composite was 53.3%.
Catalytic Activity
The catalytic tests were carried out under the same conditions as in Example 2. The complete conversion temperature was 170° C. (
Production
The ternary alloy of Ce0.5Pd0.1Au0.4 composite was synthesized from the elements Au, Pd and Ce with a purity close to 99%. The metal alloy, after coarse grinding, was oxidized in air at a temperature of 100° C. and with a relative humidity of 100%.
Characterization
The characterization of the metal precursor and then of the powder obtained after oxidation was carried out by X-ray diffraction. The metal alloy was composed predominantly of the CeAu phase, which was the only one detected by X-ray diffraction. Given the high solubility of palladium in gold, it is consistent to find gold substituted with palladium in the CeAu phase, giving rise to the Ce(AuPd) ternary compound.
The percentage content of noble metal by weight in the composite was 51%.
Catalytic Activity
The catalytic tests were carried out under the same conditions as in Example 2. The complete conversion temperature was 170° C. (
Production
The binary alloy of Au0.5Y0.5 composition was synthesized from the elements Au and Y with a purity close to 99%. The single-phase alloy (YAu phase) was oxidized in air at ambient temperature without any prior grinding step.
Characterization
The characterization of the metal precursor and then of the powder obtained after oxidation was carried out by X-ray diffraction (
The percentage content of gold by weight in the material was 63.6%.
EXAMPLE 10 Au/ZrO2/Y2O3 CompositeProduction
The ternary alloy of Zr0.75Y0.25Au composition was synthesized from the elements Au, Zr and Y with a purity close to 99%. The metal alloy, after coarse grinding, was oxidized in air at ambient temperature.
Characterization
The characterization of the metal precursor and then of the powder obtained after oxidation was carried out by X-ray diffraction. The metal alloy was made up predominantly of the ZrAu and YAu phases. Among impurity phases (<10%) no compound could be identified.
The morphology of the powder obtained was confirmed by the high value of the specific surface area (56.6 m2/g) measured by the BET method.
The percentage content of gold by weight in the composite was 62%.
Catalytic Activity
The catalytic tests were carried out under the same conditions as in Example 2.
In the case of selective CO oxidation in the presence of hydrogen, the maximum conversion is obtained at close to 60° C. (
Production
The ternary alloy of Ti0.15Ce0.65Au0.20 composition was synthesized from the elements Au, Ti and Ce with a purity of close to 99%. The metal alloy, after coarse grinding, was oxidized in air at a temperature of 80° C.
Characterization
The characterization of the metal precursor and then of the powder obtained after oxidation was carried out by X-ray diffraction. The metal alloy was composed predominantly of the Ce2Au and Ce phases, with the TiAu phase as impurity.
The percentage content of gold by weight in the composite was 24.1%.
Catalytic Activity
The catalytic properties were measured for a reaction mixture consisting of 2% CO and 2% O2 in helium with a flow rate of 50 ml/min in the case of simple oxidation and 2% CO, 2% O2 and 48% H2 in helium with a flow rate of 50 ml/min in the case of selective oxidation of CO in the presence of hydrogen. The tests were performed with 6 mg of catalyst.
The results obtained in simple oxidation (
Production
The quaternary alloy of the Zr0.125Ti0.125Ce0.25Au0.5 composition was synthesized from the elements Au, Zr, Ti and Ce with a purity close to 99%. The metal alloy, after coarse grinding, was oxidized in air at ambient temperature.
Characterization
The characterization of the metal precursor and then of the powder obtained after oxidation was carried out by X-ray diffraction (CuKα).
The alloy was a multi-phase alloy and comprised predominantly the CeAu and Ce9Au11 phases in which zirconium and titanium may partially substitute for the cerium.
The percentage content of gold by weight in the composite was 59%.
EXAMPLE 13 Au/ZrO2/TiO2/Sm2O3/CeO2 CompositeProduction
The quinary alloy of Zr0.125Ti0.125Sm0.125Ce0.125Au0.50 composition was synthesized from the elements Au, Zr, Ti, Sm and Ce with a purity close to 99%. The metal alloy, after coarse grinding, was oxidized in air at ambient temperature.
Characterization
The characterization of the metal precursor and then of the powder obtained after oxidation was carried out by X-ray diffraction. The metal alloy was a multi-phase alloy and comprised predominantly the (Ce,Sm)Au phase. The other phases present were not identified.
The percentage content of gold by weight in the composite was 58.9%.
Comparative Example 14 TiAu AlloyProduction
The single-phase α-TiAu phase was synthesized.
Characterization
The thermogravimetric analysis of the specimen showed that the TiAu phase was not oxidized (
Production
The Ce0.5Ti0.5Au alloy was synthesized.
Furthermore, to measure the influence of the choice of oxidation temperature and of the duration of treatment on the size of the nanoparticles, a Ce0.5Ti0.5Au specimen was oxidized at 400° C. in air for 1 h 30.
Characterization
The thermogravimetric analysis (
Ce0.5Ti0.5Au+020.5CeO2+0.5TiO2+Au
The analysis by X-ray diffraction of the specimen obtained after thermogravimetry showed that the TiAu phase was not oxidized (
The presence of the TiAu metal phase in the oxidized specimen, in the absence of Ce/Au binary phases (which are very reactive with respect to oxygen) in the starting metal alloy, confirms that only a ternary phase (or several ternary phases) was oxidized.
Furthermore, as illustrated in
The percentage content of gold by weight in the composite was 61%.
Comparative Example 16 TiPd AlloyProduction
The single-phase α-TiPd phase was synthesized.
Characterization
The thermogravimetric analysis (
Production The Ce0.5Ti0.5Pd alloy was synthesized.
An additional heat treatment step was carried out so as to coalesce the palladium particles of the specimen, by subjecting the specimen to a temperature of 1000° C. for 15 days.
Characterization
The X-ray diffraction analysis of the as-melted specimen exhibited diffraction peaks (intensity >20%) belonging to no binary phase, nor to any pure element nor to any of the two oxides CeO2 and TiO2 (
The thermogravimetric analysis (
Ce0.5Ti0.5Pd+O20.5CeO2+0.5TiO2+Pd.
The X-ray diffraction analysis (
The additional heat treatment carried out on this same alloy demonstrated the presence of pure palladium, with a size of 60 nm, as illustrated in
Thus, since the palladium particles are of subnanoscale size, it is possible to choose, in the case of a palladium-based nanocomposite, the size of the nanoparticles within a very extended range: from subnanometer to several tens of nanometers. By studying the growth rate of the palladium particles it was possible to define the optimum parameters for obtaining the desired nanocomposite.
The percentage content of palladium by weight in the composite was 45.8%.
Comparative Example 18 ZrPt AlloyProduction
The Zr0.5Pt0.5 alloy was synthesized.
Characterization
The thermogravimetric analysis (
Production
The Zr0.5Ce0.5Pt alloy was synthesized.
Characterization
The X-ray diffraction pattern for the Zr0.5Ce0.5Pt specimen before oxidation shows that the metal alloy is a multiphase alloy. Only two binary phases listed in the literature were identified, namely ZrPt and Zr9Pt11 (
The thermogravimetric analysis of the Zr0.5Ce0.5Pt alloy (
Zr0.5Ce0.5Pt+O20.5CeO2+0.5 ZrO2+Pt.
Owing to the low oxygen affinity of the Zr/Pt binary phases and the fact that the percentage degree of conversion (90%) of the metal alloy is close to the proportion of ternary phase present in the specimen, it therefore appears that the presence of a cerium-based ternary intermetallic compound allowed the Zr0.5Ce0.5Pt alloy to be oxidized at a temperature close to 250° C.
The thermogravimetric analysis was continued up to 800° C. The rate of oxidation increased, leading to 90% conversion in 1 h 30. This same degree of oxidation may also be achieved after several hours at the oxidation start temperature (250° C.).
The X-ray diffraction pattern for the specimen after thermogravimetry (
The percentage platinum content by weight in the composite was 56.9%.
EXAMPLE 20 Composite 2 of the Pt/CeO2/ZrO2 TypeProduction
A Zr0.75Ce0.25Pt alloy having a proportion of ternary phase less than that of the previous alloy Zr0.5Ce0.5Pt, was synthesized.
Characterization
The X-ray diffraction analysis of the specimen before oxidation showed that the alloy was a three-phase alloy and composed of a ternary phase and ZrPt and Zr9Pt11 phases (
The thermogravimetric analysis of the as-melted specimen (
Zr0.75Ce0.25Pt+O20.75 ZrO2+0.25 CeO2+Pt.
The X-ray diffraction analysis of the oxidized specimen resulting from the thermogravimetric analysis showed the presence of pure platinum and of CeO2 and ZrO2 oxides, thereby indicating the transformation of a phase containing Zr, Ce and Pt, and therefore a ternary phase. The analysis also showed that the presence of the ZrPt and Zr9Pt11 binary phases were not oxidized (
The percentage content of platinum by weight in the composite was 59%.
Thus, Examples 14 to 20 show that the process described in document FR 2 779 666 does not lead to the expected results for TiAu, TiPd and ZrPt binary compounds. It is demonstrated that the presence in these metal alloys of a ternary phase containing a rare earth, in this case cerium, makes it possible to form a composite comprising noble metal nanoparticles and oxide nanoparticles.
EXAMPLE 21 Composite 1 of Au/PrOx TypeProduction
The Pr9Au11 alloy was synthesized from the elements Au and Pr with purity close to 99%. The synthesis was carried out in an inert atmosphere in a water-cooled crucible so as to prevent the alloy from being contaminated. The alloy was then coarsely ground, so as to obtain a particle size varying from a few tens of microns to a few hundred microns, and then oxidized by exposure to air with 60% relative humidity, at a temperature of 50° C.
Characterization
The precursor metal alloy (Pr9Au11) was characterized by X-ray diffraction. The alloy was composed predominantly (>90%) of the Pr3Au4 phase of hexagonal structure (Pu3Pd4 type). The powder obtained after oxidation was characterized by X-ray diffraction (
The micrograph of
The percentage content of gold by weight in the composite was 58.2%.
Catalytic Activity
The catalytic activity of the powder was obtained by measuring the degree of CO conversion as a function of temperature.
Production
The Pr3Au4 alloy was synthesized from the elements Au and Pr with a purity of 99%. The synthesis was carried out in an inert atmosphere in a water-cooled crucible so as to prevent the alloy from being contaminated. The alloy was then coarsely ground, so as to obtain a particle size varying from a few tens of microns to a few hundred microns, and then oxidized by exposure to air with 60% relative humidity, at a temperature of 50° C.
Characterization
The precursor metal alloy (Pr3Au4) was characterized by X-ray diffraction. The alloy was composed only of the Pr3Au4 phase of hexagonal structure (Pu3Pd4 type). The powder, obtained after oxidation, was characterized by X-ray diffraction (
The micrograph of
The percentage content of gold by weight in the composite was 60.3%.
EXAMPLE 23 Au/Sm2O3 CompositeProduction
The Sm3Au4 alloy was synthesized from the elements Au and Sm with a purity close to 99%. The synthesis was carried out in an inert atmosphere by melting in an arc furnace in a water-cooled crucible so as to prevent the alloy from being contaminated. The alloy was then coarsely ground, so as to obtain a particle size varying from a few tens of microns to a few hundred microns, and then oxidized by exposure to air with 60% relative humidity, at a temperature of 50° C.
Characterization
The precursor metal alloy (Sm3Au4) was characterized by X-ray diffraction. The alloy was composed only of the Sm3Au4 phase of hexagonal structure (Pu3Pd4 type). The powder, obtained after oxidation, was characterized by X-ray diffraction (
The micrograph of
The percentage content of gold by weight in the composite was 60.1%.
EXAMPLE 24 Pt/PrOx CompositeProduction
The Pr3Pt4 alloy was synthesized from the elements Pt and Pr with a purity close to 99%. The synthesis was carried out in an inert atmosphere in a water-cooled crucible so as to prevent the alloy from being contaminated. The alloy was then coarsely ground, so as to obtain a particle size varying from a few tens of microns to several hundred microns, then heated to 800° C. from ambient temperature at a rate of 10° C./min in air, and then cooled down to ambient temperature at 30° C./min (
Characterization
The precursor metal alloy (Pr3Pt4) was characterized by X-ray diffraction. The peaks of the predominant phase (>90%) were indexed on the hexagonal structure (Pu3Pd4 type).
The thermogravimetric analysis showed that the rate of oxidation of the metal alloy increased at around 300° C. and reached its maximum at about 480° C. (
The powder, obtained after oxidation carried out by thermogravimetry, was characterized by X-ray diffraction (
The optical micrograph (
The percentage platinum content by weight in the composite was 61.2%.
EXAMPLE 25 Au/Nd2O3 CompositeProduction
The Nd3Au4 alloy was synthesized from the elements Au and Nd with a purity close to 99%. The synthesis was carried out in an inert atmosphere in a water-cooled crucible so as to prevent the alloy from being contaminated. The alloy was then coarsely ground, so as to obtain a particle size varying from a few tens of microns to a few hundred microns, and then oxidized in air at 50° C.
Characterization
The powder, obtained after oxidation carried out by thermogravimetry, was characterized by X-ray diffraction (
The optical micrograph (
The percentage content of gold by weight in the composite was 61%.
Claims
1. Process for producing a material comprising a mixture of noble metal nanoparticles and rare-earth oxide nanoparticles, comprising the following successive steps:
- a) production of a metal alloy comprising at least one noble metal selected from the group consisting of the elements Ru, Rh, Ir, Ag, Au, Pd, Pt, Ni and Cu and at least one rare earth selected from the group consisting of the elements La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y and Sc, said alloy containing a crystalline phase the rare earth content of which is greater than 10 at % and the noble metal content of which is between 25 and 75 at %; and
- b) oxidation, in an oxidizing atmosphere, of the metal alloy obtained during step a).
2. Process according to claim 1, wherein the alloy produced during step a) further includes at least one transition metal selected from the group consisting of the elements of column IVB of the Periodic Table of the Elements: Ti, Zr, Hf, from column VB; V, Nb and Ta from column VIB; Cr, Mo and W from column VIIB; Mn, Tc and Re from column IIB; and Zn, Cd and Hg, and the elements Fe, Co and Os.
3. Process according to claim 1, wherein, in the alloy produced during step a), the rare earth is partially replaced with an element of the actinide family, selected from the group consisting of Ac, Th and Pa.
4. Process according to claim 1, wherein step a) is carried out by melting the pure elements.
5. Process according to claim 1, wherein step a) is carried out by powder metallurgy or by thin films heated to a temperature of 200° C. or higher.
6. Process according to claim 1, wherein step a) is carried out by mechanical synthesis from the pure elements or from alloys.
7. Process according to claim 1, wherein, when step a) is carried out at a temperature above 50° C., it is performed in an inert or reducing atmosphere.
8. Process according to claim 1, wherein step b) is carried out at a temperature below 800° C.
9. Process according to claim 1, wherein step b) is carried out at ambient temperature.
10. Process according to claim 1, wherein step b) is performed in air.
11. Process according to claim 1, wherein the process includes, between steps a) and b), a heat treatment step for heating the metal alloy to a temperature between 200° C. and 1000° C. in an inert or reducing atmosphere.
12. Process according to claim 1, wherein the process includes, between steps a) and b) a step of grinding the metal alloy.
13. Process according to claim 1, wherein the process includes, after step b), a step of mechanically or ultrasonically grinding the powder obtained.
14. Process according to claim 1, wherein the process includes, during or after step b), a coalescence heat treatment step intended to adjust the size of the particles obtained.
15. Composite comprising a mixture of nanoparticles of at least one noble metal selected from the group consisting of the elements Ru, Rh, Ir, Ag, Au, Pd, Pt, Ni and Cu, and nanoparticles of at least one rare-earth oxide, said rare earth being selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y and Sc, said nanoparticles having a particle size of less than 20 nm, wherein the percentage content by weight of noble metal in said composite is equal to or greater than 20%.
16. Composite according to claim 15, it further comprising nanoparticles of at least one transition metal oxide, said transition metal being selected from the group consisting of the elements of column IVB of the Periodic Table of the Elements: Ti, Zr, Hf, from column VB; V, Nb and Ta from column VIB; Cr, Mo and W from column VIIB; Mn, Tc and Re from column IIB; and Zn, Cd and Hg, and the elements Fe, Co and Os.
17. Composite according to claim 15, further comprising nanoparticles of at least one oxide of an element from the actinide family, said element being selected from the group consisting of Ac, Th and Pa.
18. Use of a composite according to claim 15 for catalysis.
19. Use of a composite according to claim 15 for the manufacture of nonlinear optical instruments.
20. Use of a composite according to claim 15 for the production of nanoscale oxide powders involved in the manufacture of sintered ceramics.
Type: Application
Filed: Dec 15, 2006
Publication Date: Dec 3, 2009
Inventors: Marc-Charles Lomello-Tafin (Argonay), Jean-Luc Rousset (Lyon), Jean-Michel Moreau (Annecy), Abdelouahed Ait Chaou (Annecy Le Vieux), Franck Morfin (Loire Sur Rhone)
Application Number: 12/097,713
International Classification: B01J 23/10 (20060101); C09K 3/00 (20060101); C04B 35/00 (20060101); C23C 16/00 (20060101); B22F 3/02 (20060101); C23C 8/10 (20060101);